Braking of a DC motor in a power tool is typically accomplished by closing brake contacts across the motor windings after the trigger of the switch that switches power to the motor windings is released. This technique presents a number of problems. It results in very high currents during braking. It may also result in demagnetization of the field magnet as well as accelerating brush wear. It also results in sudden braking, which can be detrimental to the life of the tool transmission. The brake times are also not controllable.
An alternate approach is to connect power resistors across the motor windings during braking. This approach also presents problems. The power resistors are bulky and generate heat. The power resistors typically cannot be packaged in the same package as the switch that switches power to the motor windings, which houses the components for switching power on and off to the motor windings during normal tool operation. Even though the brake currents are lower, the brake times are not controllable by this approach.
FIG. 1 shows a prior art motor control circuit 10 for controlling power to a motor 12 in a cordless power tool electrical system 14 (shown representatively by dashed box 14). Cordless power tool electrical system 14 is illustratively a variable speed system, such as would be used in a variable speed drill. Motor 12 illustratively has a permanent magnet field and a wound armature. Motor control circuit 10 includes a power switch 16, illustratively a trigger switch, having main power contacts 18, braking contacts 20 and bypass contacts 22. Main power contacts 18 and braking contacts 20 are linked so that they operate in conjunction with each other. Main power contacts 18 are normally open and braking contacts 20 are normally closed and both are break-before-make contacts. The normally open side of main power contacts 18 is connected to the negative terminal of a battery 24 and the common side of main power contacts 18 is connected to controller 26 of motor control circuit 10. Motor control circuit 10 also includes run power switching device 28 and free wheeling diode 30.
Run power switching device 28 is illustratively a N-channel MOSFET with its gate connected to an output of controller 26, its source connected to the common side of main power contacts 18 and its drain connected the common side of braking contacts 20 of trigger switch 16, to one side of the windings of motor 12 and to the anode of diode 30. As is known, MOSFETs have diodes bridging their sources and drains, identified as diode 32 in FIG. 1. The other side of braking contacts 20 is connected to the positive side of battery 24 as is the other side of the windings of motor 12 and the cathode of diode 30. Since motor 12 is illustratively a wound armature/permanent magnet field motor, the motor windings to which the drain of run power switching device 28 and the positive side of battery 24 are connected are the armature windings.
Controller 26 is illustratively a pulse width modulator that provides a pulse width modulated signal to the gate of run power switching device 28 having a set frequency and a variable duty cycle controlled by a variable resistance. The variable resistance is illustratively a potentiometer 19 mechanically coupled to trigger switch 16. In this regard, controller 26 can be a LM 555 and potentiometer, the LM 555 configured as a pulse width modulator having a set frequency and a variable duty cycle controlled by the potentiometer that is mechanically coupled to trigger switch 16.
In operation, trigger switch 16 is partially depressed, opening braking contacts 20 and closing, a split second later, main power contacts 18. This couples power from battery 24 to controller 26, to the source of run power switching device 28 and to bypass contacts 22 (that remain open at this point). Controller 26 generates a pulse width modulated signal at the gate of run power switching device 28, cycling it on and off. Run power switching device 28 switches power on and off to the windings of motor 12 as it cycles on and off. The duty cycle of the pulse width modulated signal, that is, how long it is high compared to how long it is low, provided at the gate of run power switching device 28 is determined by how far trigger switch 16 is depressed. (How far trigger switch 16 is depressed determines the variable resistance of the potentiometer 19 mechanically coupled to it that provides the variable resistance used to set the duty cycle of controller 26.) The duty cycle of the pulse width modulated signal determines the speed of motor 12. As trigger switch 16 is depressed further, bypass contacts 22 close, typically when trigger switch 16 is depressed to about the eighty percent level. When bypass contacts 22 close, power is connected directly from the battery 24 to the motor windings and the variable speed control provided by controller 26 and run power switching device 28 is bypassed. Motor 12 then runs at full speed.
Diode 30, known as a free wheeling diode, provides a path for the current in the windings of motor 12 when run power switching device 28 switches from on to off. Current then flows out of the motor windings at the bottom of motor 12 (as oriented in FIG. 1) through diode 30 and back into the motor windings at the top of motor 12 (as oriented in FIG. 1).
When trigger switch 16 is released to stop motor 12, main power contacts 18 of trigger switch 16 open with braking contacts 20 closing a split second later. (Bypass contacts 22, if they had been closed, open as trigger switch 16 is being released.) Closing braking contacts 20 shorts the motor windings of motor 12, braking motor 12.
Where the cordless power tool is not a variable speed tool, such as a saw, controller 26, run power switching device 28, bypass contacts 22 and diode 30 are eliminated. Braking contacts 20 operate in the same manner described above to brake motor 12.
Where the power resistor approach is used, a power resistor is connected in series with braking contacts 20.
Controller 26 and run power switching device 28 are illustratively packaged in the same package as trigger switch 16, as may be diode 30.